The present invention relates to a composite wear component produced by casting and consisting of a metal matrix whose wear face comprises inserts which have good wear resistance properties.
The invention relates especially to wear components employed in plants for grinding, crushing and conveying various abrasive materials which are encountered in industries such as cement factories, mines, metallurgy, electricity generating stations or various quarries. These components are often subjected to high mechanical stresses in the bulk and to a high wear by abrasion at the working face. It is therefore desirable that these components should exhibit a high abrasion resistance and some ductility, to be able to withstand the mechanical stresses such as impacts and to be capable optionally of being machined.
Given that these two properties are difficult to reconcile with one another in the same material, composite components have already been proposed which have a core made of relatively ductile alloy in which isolated inserts which have a good wear resistance are embedded.
Document EP-A-0476496 proposes this technique for the production of grinding wheels whose working face has set-in inserts made of chrome pig iron.
Since it is known that ceramic materials have good abrasion resistance properties, it is also known to employ these materials for improving the abrasion resistance of wear components.
Document EP-A-0575685 proposes the use of ceramic materials in a moulding by lost-wax precision casting of small wear components.
This well-known process employs wax models which must be melted to obtain the mould cavity which must be filled with metal; this mould itself is made of ceramic and not of a conventional sand.
According to this document a ceramic pad (wafer core) is formed first, with a spongy structure which has a three-dimensional network of open pores all of which communicate with one another. This ceramic pad is formed by pouring grains of ceramic materials into an appropriate mould and, next, a liquid adhesive with a good fluidity, for example a liquid resin which, after curing, retains the grains to form the ceramic structure. The ceramic material may consist of aluminium oxide or of zirconium oxide. After having been pre-impregnated with wax, this pad is placed in a mould intended to produce the wax model of the component. The wax model is then cast and, lastly, the ceramic mould is produced by dipping the wax model in a ceramic slurry. The ceramic mould containing the wax model is then heated so as to melt the wax model. The wax thus flows from the ceramic mould but the pads inserted beforehand in the wax model remain adhesively bonded to the walls of the ceramic mould.
For the casting of metal in the ceramic mould the latter is preheated to a temperature of the order of 1150xc2x0 C., generally under vacuum.
This known technique is limited, however, to lost-wax precision moulding. Moreover, the compatibility between the metal matrix and the ceramic structure, especially in terms of temperature behaviour, presents hardly any problems in the case of the applications mentioned in this document, given that, when the metal is being cast, the mould and the ceramic structure are preheated to a high temperature. In addition, the technique is limited to the production of very precise special components, which are sold at a very high price because the lost-wax moulding process itself is very costly.
The document xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d (1985), W. Gerhartz, VCH Verslags-gesellschaft, 5th Edition XP002023826, page 5 mentions compositions based on Al2O3-ZrO2 for grinding devices intended for conditioning cast products (billets and slabs).
The technique described above cannot as such be adapted to the manufacture of wear components of larger dimensions for applications such as those encountered in plants for grinding, crushing or conveying abrasive materials, where the components generally have sections of at least 25 mm and often larger than 40 mm.
In addition, in accordance with the technique of the present invention it is not possible to cast, or at least it is difficult to envisage casting, components with thin sections, for example of less than 25 mm, because neither the mould nor the ceramic insert is preheated to high temperature before the metal is cast.
Furthermore, the component usually undergoes a subsequent heat treatment. There must therefore be some compatibility from the viewpoint of temperature behaviour between the ceramic material and the metal, to avoid cracking due to the thermal shocks when liquid metal is being cast over the ceramic inserts, and those that can be produced during the subsequent heat treatment and caused by the different expansion coefficients of both these materials.
It is necessary, furthermore, that the mechanical properties of the ceramic material should be adapted to those of the metal in order to produce a component whose properties correspond to the requirements of the specific application for which it is intended.
The aim of the present invention is to provide a composite wear component with ceramic inserts satisfactorily corresponding to the requirements listed above.
A second problem arises from the fact that, above a thickness of 25 mm of the ceramic material, poor infiltration of the metal is observed. Another objective of the present invention is to solve this second problem by proposing specific geometries of the composite wear component.
To meet the first objective the invention proposes a composite wear component produced by conventional or centrifugal casting. It consists of a metal matrix whose wear surface comprises inserts which have good abrasion resistance properties, these inserts being made of a ceramic material, itself composite, consisting of a solid solution or homogeneous phase of 20 to 80% of Al2O3 and 80 to 20% of ZrO2, the percentages being expressed by weights of constituents.
The ceramic material may additionally contain other oxides whose proportion by weight does not exceed 3 to 4%.
According to a first preferred embodiment of the present invention the composition of the ceramic material is the following:
55-60% by weight of Al2O3, and
38-42% by weight of ZrO2.
According to another preferred embodiment the composition of the ceramic material is the following:
70-77% by weight of Al2O3, and
23-27% by weight of ZrO2.
The content of ceramic materials in the insert is between 35 and 80% by weight, preferably between 40 and 60% and advantageously of the order of 50%.
This composite ceramic material is produced from an aggregate of ceramic grains which have a particle size within the range F6 to F22 according to the FEPA standard, that is to say a diameter of between approximately 0.7 mm and 5.5 mm. These ceramic grains are manufactured in a conventional way, by electrofusion, by sintering, by flame spraying or by any other process allowing the two constituents to fuse.
The ceramic grains are aggregated with the aid of an adhesive, the proportion of which does not exceed 4% by weight relative to the total weight of the pad and is preferably between 2 and 3% by weight. This adhesive may be inorganic or organic. An adhesive based on a silicate or an adhesive which is in the form of epoxy resin may be mentioned by way of example.
The invention is based on the finding that aluminium oxide (corundum) and zirconium oxide have relatively different properties and this makes it possible, by a judicious choice within the abovementioned ranges, to adjust the hardness, the toughness and the thermal expansion coefficient of the ceramic composite so as to combine a good hardness and a good toughness; and to make it compatible with the precise application for which the component is intended, on the one hand, and to obtain, on the other hand, an expansion coefficient of the composite ceramic which is close to that of the casting metal chosen, that is to say of the pig iron or of the steel which has an expansion coefficient of between 10xc3x9710xe2x88x926 and 11xc3x9710xe2x88x926.
Zirconium oxide has the advantage of having an expansion coefficient which is close to that of the metal. In addition, it contributes to good toughness, that is to say that it reduces the risks of breakage.
Aluminium oxide, for its part, contributes to good hardness. Within the pads the zirconium particles present in the alumina make it possible to increase the resistance of the latter to cracking and thus to obtain a toughness greater than that of each of the components considered in isolation, namely ZrO2 or Al2O3.
In other words, in the wear components which are subjected to strong abrasion it is advantageous to increase the proportion of aluminium oxide, not exceeding, however, a certain limit beyond which the abrasion resistance and the toughness begin to decrease. In this case the second range is rather chosen for the ceramic composition.
On the other hand, in the case of the components which are subjected to considerable impacts or to high pressures, it is advantageous to give preference to the expansion coefficient at the expense of hardness and to increase the proportion of zirconium oxide in order to decrease the stresses in the component and, consequently, the risks of breakage.
In the case of the components where there is a risk of cracking during the casting or during the subsequent heat treatment it is also advantageous to increase the proportion of zirconium oxide, to bring the expansion coefficient of the insert nearer to that of the metal matrix.
The choice of the proportions of the constituents of the composite ceramic insert may, of course, also take into account the composition of the casting metal with a view to the properties required by the application for which the component is intended. Similarly, the choice of the composition of the casting metal may be adapted to the nature of the composite insert.
Various geometries are proposed within the scope of the present invention in order to solve the problem of the poor infiltration of the liquid metal within the ceramic phase.
In the particular case where the thickness of the pad made of ceramic material becomes considerable, two or more superposed pads made of ceramic material will be proposed, according to a first embodiment, these being kept separated by a minimum gap of the order of 10 mm in order to permit the arrival of the liquid metal. This makes it possible thus to obtain correct infiltration of the various pads. In this way an appreciable increase in the proportion of the ceramic phase within the insert is obtained without being confronted with the problem of the poor infiltration by the metal.
According to another embodiment it will be proposed to produce the pad rather in the form of a xe2x80x9choneycombxe2x80x9d structure which includes various elementary cells exhibiting a polygonal or circular shape within the ceramic phase. The thickness of the walls of the various cells constituting the ceramic phase preferably varies between 5 and 25 mm.
Once again, this embodiment makes it possible to increase the quantity of the ceramic phase without, however, risking the problem of poor infiltration of the liquid metal in the case of a component whose wear takes place more particularly in depth.
Once again, the advantage lies in the fact that the walls do not exceed the limiting thickness for infiltration of the liquid metal, which is approximately 25 mm, but with a height that is practically equal to the height of the composite component. In addition, on proposing this second embodiment of the pad in the xe2x80x9choneycombxe2x80x9d form, the improvement in the grinding process is observed. In fact, after a certain period of service, hollow pits are created in the cellular metal part, which then fill with material to be ground and thus ensure that they play a part in self-protection against wear. This profile advantageously makes it possible to avoid the creation of preferential wear paths by the ground material, reflected in a drop in output rate in the case of the mills. It is noted, moreover, that this structure in the xe2x80x9choneycombxe2x80x9d form according to the second preferred embodiment makes it possible to reduce the risk of propagation of the cracks that could develop in the infiltrated pad during the production of the component. In fact, the splits which might be formed are then closed onto themselves and do not propagate within the whole component.